1. Field of the Invention
The present invention relates to the field of Doppler ultrasound imaging in living tissue. Specifically, this invention relates to an apparatus and method for display of ultrasonic data upon a video display screen for observation and diagnosis by medical personnel.
2. Prior Art
Images of living organisms typically utilize methods that pass various types of radiation through the body of the animal and measure the output with a suitable detector. For instance, x-ray images are generated by producing x-rays external to the body, passing the x-radiation through the body and observing shadows produced on x-ray sensitive film. Ultrasonic images, in contrast, are formed by producing ultrasonic waves using a transducer, passing those waves through the body, and measuring the properties of the scattered echoes from reflections inside the body using a receptor. Ultrasonic imaging apparatus may be distinguished from other medical imaging apparatus in the respect that they allow the display of soft tissues within the body which show various structural details such as organs and blood flow.
An ultrasonic imaging apparatus utilizes a probe which contains elements for transmitting Doppler pulses throughout tissue. This probe typically also contains receiving circuitry which allows reception of the reflected Doppler pulses. Some of these probes comprise a plurality of elements arranged in a linear fashion such that each of the elements can be fired at various time intervals to focus on specific parts of the body. In other systems, multiple elements are simulated by means of a moveable mechanical element within the probe wherein the Doppler pulses are transmitted at various intervals along an axis, thus simulating a plurality of elements in the probe. Each reflective pulse from the Doppler pulses emitted may then be received by a receiving unit located in the probe and transmitted to circuitry within the ultrasound apparatus for processing and generation of a display. This display, known as a b-mode image or two dimensional image of blood flow velocity, may then be generated by the apparatus and displayed on a video monitor for diagnosis and examination by an attending operator or physician.
The basic principle used in applying the Doppler method for ultrasonic imaging in a pulsed Doppler ultrasound apparatus is described as follows. When blood flow within a living subject is subjected to ultrasonic waves, corpuscles are caused to vibrate slightly while moving and reflect those ultrasonic waves. Because of the velocity of the corpuscles the frequency of the reflected waves changes from that of the transmitted waves due to the Doppler effect. The frequency shift may be detected and the amount of the shift may be displayed on a video screen for imaging blood flow in the living subject. Since the amount of shift of the transmitted waves is in relation to the blood flow velocity, the amount of blood flow and the speed of the blood flow may be observed. Noise and other signals (clutter) which have Doppler shift but don't represent blood movement in the body are filtered out. The image produced will then only represent that which is in motion. This Doppler shift frequency information is then used as blood flow information for forming a two-dimensional image or profile of the blood flow velocity.
One such apparatus used in displaying information obtained from ultrasonic pulses transmitted in the human body is shown in FIG. 1 as imaging system 100. Imaging system 100 generally comprises a probe 101 which is coupled via line 110 to transmitter/receiver circuitry 102. Transmitter/receiver circuitry 102 is designed so that the elements in probe 101 will be fired at specified time intervals, with reflective pulses being detected using probe 101 at another given time interval. Transmitter/receiver circuitry 102 is coupled to a control unit 109 via bus 120. Control unit 109 controls all circuitry in the imaging system via bus 120. Control unit 109 is further coupled to a keyboard 125 and a mouse, trackball or other device 126 for movement and control of information shown on video display 130.
Once a pulse is received by transmitter/receiver 102, such information is transmitted by line 111 to RF (radio frequency) processor 103 for further processing. This radio frequency information is further transmitted via line 114 to a graphic processor 105 and to a Doppler processor 106 via lines 114 and 113 for generation of black and white ultrasound information on video display 130. Information generated by Doppler processor 106 via in-phase (I) and quadrature (Q) signals output from RF processor 103 are transmitted via line 115 to graphics processor 105. Graphics processor 105 then integrates information received from RF processor 103 and Doppler processor 106 and then transmits scan line information to video processor 108 via line 116. In addition to information passed to graphics processor 105 and Doppler processor 106, RF processor 103 transmits I and Q signals via line 112 to color flow processor 104. Color flow processor 104 is also controlled by control unit 109 via bus 120. Color flow processor 104 is used for detecting Doppler shift and blood flow information in living tissue, and thus transmits this information via line 117 to a color scan converter 108. Such color information is used to graphically represent on video display 130 moving blood flow in a living organism. The color scan converter is used to interpolate point scan line information obtained from color flow processor 104, and transmit that information on line 118 to video processor 120 for representation of color blood flow in the human body. Video processor 120 then utilizes information obtained from graphics processor 105 for display of black and white ultrasound information and color information obtained from color scan converter 108 to generate color ultrasound information suitable for output on a video display such as 130 via line 119. Such information may be transmitted in National Television Standards Committee (NTSC) format and thus be stored on video tape for later clinical examination by attending medical personnel.
A prior art display of color Doppler ultrasound information is shown in FIG. 2 as screen 300. Screen 300 comprises a scan area 301 wherein portion 305 of scan area 301 is represented in various colors. The remainder of 301 outside 305 shows black and white ultrasound information caused by relatively stationary tissue and/or blood flow in the body being imaged. The Doppler color flow information in area 305 is shown in colors represented on scale 310 shown on the right hand portion of screen 300. One axis 321 of scale 310 represents frequency, and the second axis 320 on scale 310 represents amplitude. The range of amplitude and frequency of each pulse is represented form zero to the maximum detectable amplitude by the ultrasound receiver of probe 101 in ultrasound system 100. Frequency information is determined by measuring the phase shift of reflected waves from the pulse repetition frequency (PRF) of the reference wave. This is done, in a manner known in the art, by determining phase shift from the PRF and direction of phase shift from the PRF using I and Q signals obtained from the reflected Doppler pulses.
The frequency information shown on scale 310 will be displayed as various colors on scan region 301 according to the colors shown in scale 310. For instance, an area shown as 304 in scan area 301 may be represented in a color defined on scale 310 as 311. This area 311 may correspond with certain amplitude and frequency ranges for the reflected Doppler pulses. Likewise, other colors may further be represented on scale 310 and correspond with areas shown in scan area 305. For instance, area 304 may be represented in a color defined as being within the frequency and amplitude range shown on scale 310 as 312, and area 302 may be within the frequency and amplitude ranges shown on scale 310 as area 313. In this manner of the prior art shown as screen 300 in FIG. 2, speed and amount of blood flow information in a living organism may be clearly shown on a two-dimensional video display screen 130 for analysis.
One additional feature of the prior art system is that certain blood flow may be represented as moving towards probe transducer 101 shown in FIG. 1, and other blood flow will be shown as traveling away from probe 101 depending on the scale displayed as 310 of screen 300. For instance, one point on scale 310 such as 325, may be an origin representing blood flow with zero phase shift (zero velocity). Any regions shown in a color represented in area 326 of scale 310 may be represented as going away from the transducer, and areas shown in colors represented in area 327 may be represented as traveling towards the transducer. These colors, of course, are dependent upon whether the frequencies show a positive or negative Doppler shift from the PRF.
The choosing of a PRF is dependent upon the depth of the scan being performed, and the amount of frequency resolution required by the operator. For instance, the greater the PRF, the greater the amount of frequency resolution in the color-flow image, however, the shallower the depth of the scan. In a typical ultrasonic imaging system, each color sample volume (CSV--a horizontal row on a display), may range from approximately 0.5 millimeters to one centimeter. One aspect of the PRF is that motion of extremely high rates towards or away from the transducer generates reflected Doppler signals which are incorrectly represented on screen 300. These reflected Doppler signals may appear to be in motion away from the transducer when the blood flow is actually towards the transducer. These errors generally occur when the phase shift from the PRF is greater than 180 degrees, or the reflected signal is greater than PRF/2 (or less than-PRF/2 is the negative direction). Generally, the PRF must be twice that of the maximum frequency expected to be received to prevent this error from occurring. If the frequency of the reflected wave is greater than PRF/2, then the reflected Doppler wave will be assigned to an erroneous frequency. This error in assignment is called aliasing, and the frequencies at which aliasing occurs (.+-.PRF/2) are known as the Nyquist limits. The Nyquist limits of the prior art system shown on screen 300 of FIG. 2 are represented as points 314 and 315. If point 315 is the positive Nyquist limit and 314 is the negative Nyquist limit for the PRF, certain reflected pulses which exceed the frequency 315 on scale 310 will appear in area 326 (the negative area) of scale 310. Although the Nyquist limit of the PRF is an inherent limitation in pulsed Doppler systems, the prior art color display shown as screen 300 in FIG. 2 does not clearly illustrate this aliasing error. Therefore, an improved method for displaying colors in an ultrasonic pulse Doppler imaging system is required which will clearly display the aliasing error. This allows an operator performing the scan to adjust the PRF, if desired, to minimize aliasing problems and maximize the resolution of displayed information.